1. Field of the Invention
The present invention relates to the current-voltage conversion circuit which converts the current to be detected into voltage.
2. Description of the Related Art
In regards to battery driven electronic equipment such as Portable mobile phones, PDA (Personal Digital Assistant) and portable CD players and the likes, it is necessary to manage the battery which is the drive source of the load by monitoring power consumption of the load or monitoring the level of the battery.
In that case, it uses the current-voltage conversion circuit and transforms the load current supplied to the load from the battery into voltage and is thought to be a method capable of monitoring the supplied load current from the battery based on this converted voltage. There is a document in regards to the current-voltage conversion circuit in the following.
(for example, see Japanese Patent Laid-Open No. 8-17066).
The load circuit uses the current-voltage conversion circuit as a method of converting load current into voltage, providing detection resistor to the path of the load current, and this is thought to be a method capable of monitoring the voltage drop across the detection resistor. In order for this detection resistor to function as a voltage loss, it is necessary to set a small value for this resistance value and the voltage drop becomes a very small value. In order for this kind of small voltage drop A/D conversion, it is necessary to amplify the voltage, once, by way of the operational amplifier.
However, when using an operational amplifier in amplifying voltage, offset voltage of the operational amplifier becomes a problem. With offset voltage, if there is a negative voltage shift in the amplified voltage, when the voltage drop across the resistor is larger than 0, (namely, when the load current that is detected is larger than 0), the amplified voltage may become 0. As a result, when there is little current detection supplied to the load from the battery there are times when that current cannot be detected (below, this range of current which cannot be detected is called the “undetectable current region”).